1. Field of the Invention
The present invention relates to a super-hard ceramic and, more particularly, to a ceramic which has superhardness, excellent wear resistance, high toughness and chipping resistance, and which can be used as a material for cutting tools and workpieces to be machined, or parts for high precision machining, and the like.
2. Description of the Background Art
Conventionally, alumina ceramics have widely been used as wear-resistant materials, sealing materials, machine parts, and the like, because they possess high hardness, excellent resistance to oxidation, and outstanding wear resistance, and can be produced at a relatively low cost. However, they essentially lack toughness and strength, and have the drawbacks of being apt to grow grains and to have numerous pores form in their structures. This entails a substantial limitation to their use because of brittleness, comparatively low strength and low working accuracy, and they can rarely be used as materials for cutting tools and workpieces to be machined, or parts for high precision machining, and the like.
In order to eliminate these drawbacks, some attempts have been undertaken to improve the sintered structure of alumina ceramics by adding grain-growth inhibitors or by means of hot pressing. However, these means have almost no effect in overcoming the problem of brittleness or in improving toughness.
On the other hand, a dispersed-reinforced alumina ceramic was prepared by dispersing and combining in the alumina matrix a metal carbide such as titanium carbide possessing elasticity, hardness, and strength which is equivalent to or better than alumina and capable of coexisting with alumina. Such a reinforced-alumina ceramic could provide significant improvements in strength, toughness, and grain-growth inhibitory effect over conventional alumina ceramics, and could be applied to high performance cutting tools, materials for excellent wear-resistant parts, or non-magnetic electric components particularly for magnetic heads, and the like.
However, these dispersed-reinforced alumina ceramics have problems still to be improved. Specifically, it is difficult to sinter these types of ceramics in air because titanium carbide will be oxidized. While in vacuum or a non-oxidative atmosphere, they have a tendency to form pores due to the reducing reaction so that the densification of the ceramic will not proceed. To improve sinterability of the ceramics, an attempt was made to use conventional sintering additives. However, a great amount of additives is required for this purpose, and the resulting ceramics have properties which deviate from those originally intended. On the other hand, the use of a small amount of additives sometimes requires a reheat-treatment by means of hot isostatic pressing to densify the ceramics, and accordingly tends to cause grain growth. Although it is possible to sinter the ceramics with no grain-growth using the least amount of sintering additives when direct-sintering by high-pressing during heating is applied, the resulting titanium carbide-alumina ceramics are insufficient in toughness and in chipping resistance. Specifically, when a workpiece made of these ceramics is subjected to cutting or grinding, chipping is liable to occur and when these ceramics are adopted for a cutting tool, tool chips often fracture in a heavy cutting.
In view of this situation, the present inventors have conducted extensive studies in order to develop a super-hard ceramic having the superior properties inherent in titanium carbide-alumina ceramics which possess high hardness and excellent wear resistance, as well as superior toughness and chipping resistance.
As a result, the inventors have found that an alumina ceramic possessing excellent toughness and chipping resistance without accompanying grain growth, while retaining the characteristics inherent in alumina ceramics, could be obtained by incorporating neodymium oxide and lithium oxide in addition to alumina, titanium carbide, zirconium carbide, and magnesium oxide. This finding led to the completion of the present invention.